Introduction to fatigue
1. INTRODUCTION
1.1 The nature of fatigue
Fatigue is the mechanism whereby cracks grow in a structure. Growth only occurs under fluctuating stress. Final failure generally occurs in regions of tensile stress when the reduced cross section becomes insufficient to carry the peak load without rupture. Whilst the loading on the structure is stationary, the crack does not grow under normal service temperatures. Many structures, such as building frames, do not experience sufficient fluctuating stress to give rise to fatigue problems. Others do, such as bridges, cranes, and offshore structures, where the live loading is a higher proportion of the total load.
1.2 How welds fatigue
In welded steel structures, fatigue cracks will almost certainly start to grow from welds, rather than other details, because:
- Most welding processes leave minute metallurgical discontinuities from which cracks may grow. As a result, the initiation period, which is normally needed to start a crack in plain wrought material, is either very short or non-existent. Cracks, therefore, spend most of their life propagating, i.e. getting longer.
- Most structural welds have a rough profile. Sharp changes of direction generally occur at the toes of butt welds and at the toes and roots of fillet welds (see Figure 1). These points cause local stress concentrations of the type shown in Figure 2. Small discontinuities close to these points will therefore react as though they are in a more highly stressed member and grow faster.
1.3 Crack growth history
The study of fracture mechanisms shows that the growth rate of a crack is proportional to the square root of its length, given the same stress fluctuation and degree of stress concentration. For this reason, fatigue cracks spend most of their life as very small cracks which are hard to detect. Only in the last stages of life does the crack start to cause a significant loss of cross section area, as shown in Figure 3. This behaviour poses problems for in-service inspection of structures.
